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Abstract: TH-OR077

Development of a Novel Predictive Equation for Ionized Calcium in Hospitalized Subjects: Albumin-Corrected Calcium Equation Is Extremely Inaccurate

Session Information

Category: Fluid and Electrolytes

  • 902 Fluid and Electrolytes: Clinical


  • Ramirez-Sandoval, Juan Carlos, Instituto Nacional de Ciencias Medicas y Nutricion Salvador Zubiran, México city, Distrito FEDERAL, Mexico
  • Gutierrez valle, Fabian, Universidad Panamericana, CDMX, Mexico
  • Ley, Sofia, Universidad Panamericana, CDMX, Mexico
  • Pastrana Brandes, Santiago, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City, Mexico
  • Correa-Rotter, Ricardo, Institutor Nacional de la Nutricion, Mexico City, Mexico
  • Diener Cabieses, Pablo, Universidad Panamericana, CDMX, Mexico

In clinical practice total serum Ca is often corrected according to albumin (0.8mg/dL per each 1g/L Alb<4 g/L). Recently, hidden Ca disorders have been associated to higher mortality in dialysis. Our objective was to develop a novel-specific correction equation for ionized Ca.


We reviewed electronic data from all hospitalized patients of a single tertiary-care center (2017-2018). Hidden hypocalcemia and hidden hypercalcemia were defined as: normal Alb corrected-Ca & ionized Ca <4.3 or >5.2 mg/dL respectively.


We analyzed 7,158 Ca samples from 5,618 subjects (age 54±18 y, female 55%, 44% with AKI or CKD). Hypercalcemia and hypocalcemia according to ionized Ca occurred in 3.8% (275/7158) and 28.8% (2059/7158) respectively. Alb corrected-Ca had a poor correlation with ionized Ca (r:0.56, p<0.001). Hidden hypercalcemia and hidden hypocalcemia occurred in 2.2% (160/7158) and 3.7% (271/7158) measurements respectively; 5.1% (362/7158) and 10.7%(766/7158) were erroneously diagnosed as hypercalcemia or hypocalcemia respectively when Alb corrected-Ca was employed. Agreement between Alb corrected-Ca for hypo, normo or hypercalcemia was poor (kappa 0.23).
A novel laboratory-specific prediction equation was developed: Ionized Ca (mg/dL, reference value 4.3-5.2 mg/dL)=0.44*total Ca – 0.27*Alb (g/L) – 0.06*P(mg/dL) – 0.02*CO2 (mEq/L) + 2.16.
This new equation substantially improved adjusted R2 to 0.81 (95% CI 0.78-0.82, p<0.001) when compared with Alb corrected-Ca equation (R2=0.56). Area under ROC curve for hypercalcemia and hypocalcemia diagnosis with new equation were 0.98 (95% CI 0.97-0.99, p<0.001) and 0.86 (95% CI 0.84-87, p<0.001) respectively. In univariate models, SCr and eGFR were associated with Ca-status misdiagnosis (OR:18.1, p<0.001) yet this association disappeared when multivariate analysis was adjusted to P and CO2 levels.


The novel equation proposed for prediction of ionized Ca is superior to the Alb corrected-Ca equation and could be useful when ionized Ca is not available. The conventional formulas currently used in practice are inaccurate and misclassify many patients, in particular when renal dysfunction with phosphorus or bicarbonate disturbances are present.